A variety of optical detection systems have been developed which are sensitive to radiation within various wavelength bands. For example, digital cameras are commercially available to consumers and are configured to detect and record reflections and emissions of light having a wavelength in the visible spectrum band, thereby effectively capturing and recording scenes. As another example, infrared imaging systems are provided to detect radiation in the infrared wavelength band that is emitted from or reflected by objects within a scene. Such infrared imaging systems can view objects within scenes that would normally not be apparent to an optical detection system capable only of detecting light in the visible spectrum. These systems are particularly useful for real-time viewing of scenes at night or through smoke, and are frequently utilized in military equipment for detecting missiles, aircraft, vehicles, vessels, and the like.
In use, optical systems (e.g., visible or infrared) are often subjected to undesired radiation from the sun or from another light source, for example. In conventional detectors, such undesired radiation can saturate the detector and make it difficult and sometimes impossible for the detector to distinguish objects of interest from the undesired radiation. For example, in an infrared imaging system, the amount of solar radiation can be extremely large when compared to the radiation signals available from objects to be detected, and the solar radiation can therefore saturate the infrared detector. As an example, with conventional infrared imaging systems, it can be difficult to detect an aircraft flying near the sun, as the amount of infrared radiation from the sun is large when compared to the infrared radiation from the aircraft. The solar radiation can cause blooming in the image, reduce sensitivity in the affected area, and even permanently damage the detector. Moreover, the intense solar radiation can scatter in the optical components, and such scattering can reduce or eliminate the detector's ability to show the presence of the aircraft. The sun can also cause similar problems in a system configured to detect light in the visible wavelength band. Moreover, like solar radiation, radiation from an intense artificial light source (e.g., an incandescent light) aimed in the direction of an optical detector could cause saturation of the optical detector.
Various techniques could be utilized to attempt to alleviate the saturation and blooming problems caused by solar radiation and other undesired types of radiation. For example, optical filters could be utilized to filter radiation from the undesired source. However, many such filters typically attenuate the entire image and therefore can simultaneously attenuate radiation from objects of interest, making them more difficult to detect. Moreover, electronic techniques, such as digital anti-blooming methods, could be utilized in an attempt to attenuate the effects of undesired radiation. However, purely electronic techniques typically cannot accommodate the large flux levels often present (e.g., from the sun) while still providing accurate detection of background and adjacent areas and while quickly recovering from the blooming effect. In addition, while non-linear absorbers could be utilized, such absorbers usually require extremely high photon flux in the wavelength band of interest (e.g., the infrared band), such as would be emitted from a high power laser, before their attenuating function becomes active. It is therefore desirable to provide improved methods and apparatuses for selectively limiting the effects of undesired radiation, such as solar radiation, in an optical system.